470 SCIENCE PROGRESS. 



endodermis around a vascular bundle has only a physiologi- 

 cal significance, and cannot change the homologies of the 

 surrounding tissues. Pfitzer (14) showed long ago that 

 in some Equiseta mere strands of parenchyma may be 

 surrounded by their own endodermis ; yet no one would 

 regard such strands as homologous with vascular bundles 

 or with steles. 



Van Tieghem, in his Treatise 011 Botany, has a section 

 on the return from polystelic or astelic to monostelic 

 structure (5, p. 767). In the pedicel of an Auricula, for 

 example, the separate steles first become fused laterally, 

 giving rise to the "gamostelic" condition, and next the 

 internal phloem, pericycle, and endodermis disappear, so 

 that a typical monostelic structure is assumed. Here it is 

 evident that the pith of the monostelic pedicel is the direct 

 continuation of the " internal cortical tissue " of the poly- 

 stelic part of the stem. The author warns us, it is true, 

 against confusing such stems, which have become mono- 

 stelic by reduction, with truly monostelic organs ; there is, 

 however, no distinction whatever in their structure. Cases 

 such as these warn us that the theory of polystely must not 

 be applied too rigidly, if we would avoid forced interpreta- 

 tions. 



The fundamental question now arises : Does the 

 distinction of the great anatomical regions, stele, cortex 

 and epidermis, rest on a satisfactory developmental basis ? 

 Can we trace definite histogenetic layers, whether in the 

 actual embryo, or in the growing points, from which, and 

 from which alone, the definitive histological regions are 

 respectively derived ? 



The whole subject of histogenesis is far too large to be 

 treated of in this article ; but some attempt may be made 

 to indicate the present position of the question. 



The great majority of plants differ from animals in being 

 permanently embryonic : that is to say, they retain, through- 

 out their whole life, portions of undifferentiated tissue in 

 their growing points, which continue to give rise to new 

 organs and new tissues, until the life of the whole plant is 

 at an end. Exceptions to this rule are very rare. Ludwig 



